1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a vertical alignment mode liquid crystal display device with high-speed response.
2. Description of the Prior Art
As well known in the art, a vertical alignment mode liquid crystal display was proposed to improve the viewing angle and response speed properties of a twisted nematic mode liquid crystal display.
Although not shown in the accompanying figures, in this vertical alignment mode liquid crystal display, a liquid crystal layer formed of liquid crystals with negative dielectric anisotropy is sandwiched between upper and lower electrodes, each having a liquid crystal driving electrode. Also, vertical alignment films are disposed on the inner surfaces of the upper and lower substrates, respectively. And polarizers are attached to the outer surfaces of the upper and lower substrates, respectively, in such a manner that their polarizing axes cross each other.
However, in this vertical alignment mode liquid crystal display, the liquid crystals have refractive index anisotropy due to their rod-like shape, and for this reason, the screen image of the display varies depending on viewing angle. For example, before application of an electric field, all the liquid crystal molecules are aligned in a direction perpendicular to the substrates, and thus, on the front of the screen, complete darkness is achieved but on the side of the screen, light is leaked to cause deterioration in image quality.
Thus, in order to compensate for the deterioration of image quality caused by the refractive index anisotropy of the liquid crystal molecules, there was proposed a structure wherein the electric field is distorted to align the liquid crystal molecules in four directions, thereby improving the viewing angle.
For example, U.S. Pat. No. 6,288,762 discloses a structure in which protrusions serving to distort the electric field are formed on substrates. This structure is shown in FIG. 1.
As shown in FIG. 1, a lower substrate 11 and an upper substrate 12 are disposed opposite one another while interposing liquid crystal molecules 13 therebetween. Protrusions 14 are formed on the inner surface of each of the lower substrate 1 and the upper surface 12.
In this structure, an electric field is distorted around the protrusions 14 upon its application such that the liquid crystal molecules 13 are symmetrically aligned. As a result, there are formed a multi-domain of the liquid crystal molecules. This compensates for the deterioration of image quality caused by the refractive index anisotropy of the liquid crystal molecules.
In another attempt to distort the electric field, there was proposed a method in which a liquid crystal driving electrode with slits is used in place of the protrusions. The structure of the liquid crystal driving electrode is shown in FIG. 2, and the principal of operation of this structure equals to that of the protrusions. In FIG. 2, the reference numeral 20 designates the liquid crystal driving electrode having the slits.
However, in the vertical alignment mode liquid crystal display utilizing the protrusion or slit structure, the liquid crystal molecules start to be tilted in the protrusion or silt portions upon application of the electric field and then the remaining portion of the liquid crystal molecules are also tilted with the passage of time. Due to this phenomenon, if the interval between the protrusions or slits is too long, the response time of liquid crystal molecules will be lengthened.
Thus, in order to solve this problem, a jagged liquid crystal driving electrode was proposed by Fujitsu, Co., Japan, in SID 2001 Digest, page 1066. The structure of this proposed electrode is shown in FIG. 3 where the reference numeral 30 designates the jagged liquid crystal driving electrode. This structure is known as significantly improving the response speed of liquid crystals.
However, in the vertical alignment mode liquid crystal display utilizing the jagged liquid crystal driving electrode, the response speed of liquid crystals is improved, but at a portion free from the electrode, the electric field is not generated to cause a phenomenon where the orientation of the liquid crystal molecules is delayed. For this reason, there are problems in that the response time is delayed and also the deterioration of image quality occurs due to the generation of disclination lines.
FIG. 4 is a drawing showing simulation results on a section taken along the line IV—IV of FIG. 3. From FIG. 4, it can be found that an increase in initial transmittance is insufficient at the portion free from the electrode.